For loops in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic syntax of a for loop in MATLAB?

The basic syntax is:

for index = startValue:endValue
    % code to repeat
end

This repeats the code for each value of index from startValue to endValue.

beginner

How does a for loop in MATLAB differ from a while loop?

A for loop runs a fixed number of times, determined before the loop starts.

A while loop runs as long as a condition is true, which can change during execution.

intermediate

What happens if the loop variable in a MATLAB for loop is a vector?

The loop runs once for each element in the vector, assigning the loop variable to that element each time.
Example:

for i = [2, 4, 6]
    disp(i)
end

prints 2, then 4, then 6.

intermediate

Can you modify the loop variable inside a MATLAB for loop? What happens?

Modifying the loop variable inside the loop does not affect the next iteration.
MATLAB sets the loop variable to the next value in the sequence at the start of each iteration regardless of changes inside the loop.

beginner

How do you exit a for loop early in MATLAB?

Use the break statement inside the loop.
When MATLAB encounters break, it stops the loop immediately and continues with the code after the loop.

What does this MATLAB code print?

for i = 1:3
    disp(i)
end

A Error

B 3 2 1

C 1 1 1

D 1 2 3

The loop runs from 1 to 3, printing each number in order.

In MATLAB, what happens if you write:

for i = [5 10 15]
    disp(i)
end

A It prints 5, then 10, then 15

B It prints 1 to 15

C It prints only 15

D It causes an error

The loop variable takes each value in the vector in order.

What is the purpose of the break statement in a MATLAB for loop?

A To skip the current iteration

B To stop the loop immediately

C To restart the loop

D To pause the loop

break stops the loop right away and continues after it.

If you change the loop variable inside a MATLAB for loop, what happens in the next iteration?

A The loop variable resets to the next value in the sequence

B The loop stops

C The changed value is used

D An error occurs

MATLAB resets the loop variable to the next value in the sequence regardless of changes.

Which of these is a valid for loop header in MATLAB?

A for i = 1 to 5

B for i in 1:5

C for i = 1:5

D for (i=1; i<=5; i++)

MATLAB uses for i = 1:5 syntax for loops.

Explain how a for loop works in MATLAB and give an example.

Think about how the loop variable changes each time.

Describe what happens if you modify the loop variable inside a MATLAB for loop.

Consider how MATLAB controls the loop variable each iteration.

Practice

1. What does a for loop do in MATLAB?

easy

A. Stops the program immediately

B. Runs commands only once

C. Creates a new variable

D. Repeats a set of commands a fixed number of times

Solution

1. Step 1: Understand the purpose of a for loop

   A for loop is designed to repeat commands multiple times based on a range or list.

2. Step 2: Compare options with the definition

   Only Repeats a set of commands a fixed number of times correctly describes repeating commands a fixed number of times.

3. Final Answer:

   Repeats a set of commands a fixed number of times -> Option D

4. Quick Check:

   For loop = repeat commands [OK]

Hint: For loops repeat commands multiple times [OK]

Common Mistakes:

• Thinking for loops run only once
• Confusing for loops with if statements
• Believing for loops stop the program

2. Which of the following is the correct syntax for a for loop in MATLAB?

easy

A. for i = 1:5
disp(i)
end

B. for (i = 1; i <= 5; i++)
disp(i)
end

C. loop i from 1 to 5
disp(i)
end

D. for i in range(1,5):
disp(i)

Solution

1. Step 1: Identify MATLAB for loop syntax

   MATLAB uses 'for variable = start:end' and ends with 'end'.

2. Step 2: Check each option

   for i = 1:5 disp(i) end matches MATLAB syntax. Options B, C, and D use other languages' syntax.

3. Final Answer:

   for i = 1:5\n disp(i)\nend -> Option A

4. Quick Check:

   MATLAB for loop = for i = 1:5 ... end [OK]

Hint: MATLAB for loops use 'for var = start:end' and end with 'end' [OK]

Common Mistakes:

• Using parentheses like C or JavaScript
• Using Python or other language syntax
• Forgetting the 'end' keyword

3. What is the output of this MATLAB code?

sum = 0;
for i = 1:3
  sum = sum + i;
end
disp(sum)

medium

A. 6

B. 3

C. 0

D. Error

Solution

1. Step 1: Trace the loop iterations

   The loop runs i = 1, 2, 3. sum starts at 0.

2. Step 2: Calculate sum after each iteration

   sum = 0+1=1, then 1+2=3, then 3+3=6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Add numbers 1 to 3 inside loop [OK]

Common Mistakes:

• Forgetting to initialize sum
• Adding outside the loop
• Confusing loop range

4. Identify the error in this MATLAB for loop:

for i = 1:4
  disp(i)
  i = i + 1;
end

medium

A. Missing 'end' keyword

B. Modifying loop variable inside the loop

C. Incorrect loop range syntax

D. disp function is invalid

Solution

1. Step 1: Review loop variable usage

   The loop variable 'i' is controlled by MATLAB and should not be changed inside the loop.

2. Step 2: Identify the problem

   Changing 'i' inside the loop causes unexpected behavior or errors.

3. Final Answer:

   Modifying loop variable inside the loop -> Option B

4. Quick Check:

   Do not change loop variable inside for loop [OK]

Hint: Never change the loop variable inside the loop [OK]

Common Mistakes:

• Changing loop variable inside loop body
• Forgetting 'end' keyword (not the case here)
• Misunderstanding disp function

5. How can you use a for loop in MATLAB to create a vector v containing squares of numbers from 1 to 5?

hard

A. v = [];
for i = 1:5
v = v + i^2;
end

B. for i = 1:5
v(i) = i*2;
end

C. v = zeros(1,5);
for i = 1:5
v(i) = i^2;
end

D. v = 1:5;
for i = 1:5
v(i) = i + 2;
end

Solution

1. Step 1: Initialize vector to store results

   Pre-allocate vector v with zeros to hold 5 elements.

2. Step 2: Use for loop to assign squares

   For each i from 1 to 5, assign v(i) = i squared.

3. Final Answer:

   v = zeros(1,5);\nfor i = 1:5\n v(i) = i^2;\nend -> Option C

4. Quick Check:

   Pre-allocate vector and assign squares in loop [OK]

Hint: Pre-allocate vector, assign squares inside loop [OK]

Common Mistakes:

• Not pre-allocating vector before loop
• Adding instead of assigning inside loop
• Using wrong operation for squares